Oscillators which comprise tuned circuits and amplifiers that use ac and dc feedback can have gain variations and signal clipping because the electrical characteristics of the amplifiers vary with integrated circuit (IC) manufacturing process variations and with temperature variations. An example of a prior art oscillator circuit 10 is shown in FIG. 1. Oscillator circuit 10 comprises an inverting amplifier 12 and a tuned circuit 14 which is shown within the dashed line rectangle. In one illustrative embodiment tuned circuit 14 comprises capacitors 16 and 18 and a resonator (crystal) 20. An input of amplifier 12 is coupled to first terminals of resonator 20 and capacitors 16 and to a terminal 22. An output of amplifier 12 is coupled to second terminals of resonator 20 and capacitor 18 and to a terminal 24. Second terminals of capacitors 16 and 18 are coupled together to a terminal 26, which is shown coupled to a reference voltage that is illustratively ground (zero volts).
In FIG. 2 there is shown a prior art inverting amplifier 100 which comprises a p-channel metal-oxide-semiconductor (MOS) transistor 102, an n-channel MOS transistor 104 and a resistor 106. The structure of amplifier 100 is typically used as amplifier 12 of FIG. 1. The source of transistor 102 is coupled to a terminal 108 and to a positive voltage Vdd. The source of transistor 104 is coupled to a terminal 110 and to a reference voltage which is illustratively ground (zero volts). A first terminal of resistor 106 is coupled to the gates of transistors 102 and 104 and to a terminal 112 which serves as an input terminal of amplifier 100. The drains of transistors 102 and 104 are coupled to a second terminal of resistor 106 and to a terminal 114 which serves as an output terminal of amplifier 100. Resistor 106 provides an ac and a dc feedback from the output terminal 114 to the gates of transistors 102 and 104. Variations in the IC manufacturing process used to fabricate amplifier 100 and operating temperature changes affect the gain of amplifier 100. Accordingly, it is difficult to predict the peak-to-peak output voltage of amplifier 100.
It is desirable to have an integrated circuit (IC) inverting amplifier which generates an output signal having an amplitude which is essentially insensitive to variations in the IC manufacturing process and operating temperature variations. Such an amplifier is particularly useful as a component of an oscillator circuit.